As soon as we start using a new antibiotic the clock is ticking because bacteria start evolving defenses to it, making it less and less useful. Eventually resistant bacteria will be common enough that we need to start using a new one.

Cancer isn't like this: each person's cancer is independent from everyone else's. Which means that if we develop an effective cancer drug we don't have to worry about cancers becoming resistant to it. Sure, an individual's cancer might not respond to it, but we don't have to worry about this becoming more and more common as we distribute the drug more widely.

The key difference is that cancer isn't contagious. It has no way to take what it's learned about resisting some drug and put that to use in another person. While bacteria left behind in your body after you stop a course of antibiotics early, the ones that had some innate resistance to the antibiotic you were taking, can go on to breed more resistant bacteria and infect other people.

I'm thankful that different people's cancers can't speak to each other. If they had some way to pass along information the problem would be vastly harder because we wouldn't have to solve it just once, but over and over again as old solutions became less effective.